A test case is a set of conditions or variables used to determine if a software application or system behaves as expected. It typically includes inputs, execution steps, and the expected outcome. Test cases are essential in software development to ensure that features work correctly and meet user requirements. In software testing, test cases help identify bugs and issues before the product is released. They can be manual or automated and are often documented in a test management tool. Effective test cases improve the quality of the software and enhance user satisfaction by ensuring that the application functions properly under various scenarios.